1. Field of the Invention
The present invention relates to electrophotographic printing and more specifically, this invention relates to electrophotographic printers which include a transfusing member and a system for managing the temperature distribution on the transfusing member.
2. Prior Art
Electrophotographic marking is a well known and commonly used method of copying or printing original documents. Electrophotographic marking is typically performed by exposing a light image of an original document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrophotographic latent image of the original document on the photoreceptor's surface. Examples of electrostatic formation of latent images are disclosed in U.S. Pat. Nos. 4,408,214, 4,365,549, 4,267,556, 4,160,257 and 4,155,093. Toner particles are then deposited onto the latent image so as to form a toner powder image. That toner powder image is then transferred from the photoreceptor, either directly or after an intermediate transfer step, onto a marking substrate such as a sheet of paper. The transferred toner powder image is then fused to the marking substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creations of another image.
The foregoing generally describes a typical black and white electrophotographic marking machine. Electrophotographic marking can also produce color images by repeating the above process once for each color that makes the color image. For example, the charged photoconductive surface may be exposed to a light image which represents a first color, say cyan. The resultant electrostatic latent image can then be developed with cyan toner particles to produce a cyan image which is subsequently transferred to a marking substrate. The foregoing process can then be repeated for a second color, say magenta, then a third color, say yellow, and finally a fourth color, say black. Beneficially each color toner image is transferred to the marking substrate in superimposed registration so as to produce the desired composite toner powder image on the marking substrate. Processes for forming monochromatic or polychromatic electrostatic images are disclosed, for example, in U.S. Pat. Nos. 3,672,887, 3,687,661, 4,395,472, 4,353,970, 4,403,848 and 4,286,031.
The color printing process described above superimposes the various color toner powder images directly onto a marking substrate. Another electrophotographic color printing process uses an intermediate transfer member. In systems which use an intermediate transfer member successive toner images are transferred in superimposed registration from the photoreceptor onto the intermediate transfer member. Only after the composite toner image is formed on the intermediate transfer member is that image transferred and fused onto the marking substrate.
The most common developing materials are dry powder toners. Dry powder developers are typically comprised of not only toner particles but also of carrier granules. The toner particles triboelectrically adhere to the carrier granules until the toner particles are attracted onto the latent image. An alternative to dry powder developing materials are liquid developers. Liquid developers, also referred to as liquid inks have a liquid carrier into which toner particles are dispersed. When developing with liquid developers both the toner particles and the liquid carrier are advanced into contact with the electrostatic latent image. The liquid carrier is then removed by blotting, evaporation, or by some other means, leaving the toner particles behind.
Intermediate transfer members can also be used in the fusing process. Intermediate transfer members which are used in fusing are referred to herein as transfusing members, and the combined processes of transferring and fusing is called transfusing. Transfusing is highly desirable since the size and cost of transfusing printing machines can be less than comparable printing machines which use a separate transfer station and fusing station. Other advantages such as improved image quality can also be obtained by transfusing. Transfusing members are usually pinched between one or more contact rollers and a backup roller such that a fusing pressure is created between the nip of the backup roller and the transfusing member. During fusing a marking substrate passes between the backup roller and the transfusing member and heat is applied to the toner image. The combination of heat and pressure causes the toner image to fuse onto the marking substrate. Transfusing may be done without heat, but the resulting quality is usually inferior.
One problem with transfusing members is that the transfusing member usually needs to be hot to provide high-quality fusing. The heat can damage the photoreceptor and can interfere with the transfer process. Other problems with transfusing members, are due to the fact that the application of transfuse members requires both heating and cooling of the member. Higher temperatures (e.g. 120.degree. C. to 200.degree. C.) are required for fusing, while reasonably lower temperatures (e.g. 40.degree. C.) are required for a development process or a transfer process of the image from a photoreceptor to a transfuse member. Thus, temperature distribution and fluctuations on a transfuse member is a critical issue. In view of this, an efficient means of managing the transfuse member temperature distribution would enable wider applicability of the transfuse process. Since the transfuse member may be used for imaging purposes, any sliding type of contact should be avoided to assure good motion quality. In accordance with the features and advantages offered by the present invention, a passive means of effectively moving heat from high temperature portions of the transfuse member to low temperature portions of the transfuse member thereby reducing heating and cooling requirements, is described.